1. Field of the Invention
The present invention relates to a wafer flattening process for flattening the surface of a wafer polished to a mirror surface in a previous process to a further high precision by a plasma etching apparatus, a wafer flattening system, and a wafer.
2. Description of the Related Art
FIG. 15 is a process diagram showing an example of a wafer flattening process of the related art.
In FIG. 15, reference numeral 100 is a chemical mechanical polishing (CMP) apparatus, while reference numeral 200 is a plasma etching apparatus.
In this wafer flattening process, first, in the CMP apparatus 100, a wafer W held and pressed by a carrier 101 is made to rotate in an opposite direction to a rotating platen 102 to chemically mechanically polish the surface Wa of the wafer W to a mirror surface by a polishing pad 102a of the platen 102. Suitably thereafter, the wafer W is conveyed to the plasma etching apparatus 200 where the surface Wa is turned upward and the wafer W held by a holder 201. Next, ion or radical or other activated species gas G produced in a plasma generator 202 is sprayed from a nozzle 203 to the surface Wa of the wafer W to locally etch a portion of the surface Wa thicker than the reference thickness value (hereinafter referred to as a "relatively thick portion").
Specifically, as shown in FIG. 16, the holder 201 is made to move to position where the nozzle 203 directly above a relatively thick portion of the wafer W and locally etch the relatively thick portion by the activated species gas G from the nozzle 203 to further flatten the surface Wa.
The wafer flattening process of the above related art, however, suffered from the following problems.
The activated species gas G sprayed from the nozzle 203 is a fluid and etches a substance by a chemical reaction with that substance. Accordingly, if the conditions of the region of spraying of the activated species gas G differ, the amount of etching of the substance also changes.
That is, as shown in FIG. 16, when the nozzle 203 is positioned at the inside portion of the wafer W, since the activated species gas G is sprayed under evacuated environment existing only wafer, the activated species gas G is sprayed almost symmetrically toward the center of the nozzle, thereby reacting silicon and the like of the surface Wa and etching relatively thick portion thereof at predetermined amount.
In contrast, shown in FIG. 17, when the nozzle 203 is positioned over the outer peripheral portion of the wafer W, the activated species gas G is sprayed across the outer peripheral portion of the wafer W and the holder 201 and activated species gas G causes a chemical reaction with the holder 201 formed by aluminum etc. As a result, the product A of the chemical reaction between the holder 201 and the activated species gas G deposits on the surface etc. of the outer peripheral portion and inhibits the etching of the outer peripheral portion and the amount of etching of the outer peripheral portion ends up drastically falling.
Further, as shown in FIG. 18, when there is a level difference between the surface Wa of the wafer W and the surface of the holder 201, the flow of the activated species gas G becomes disturbed at the level difference portion, more activated species gas G ends up flowing to the outside of the wafer W, and the amount of etching of the outer peripheral portion drastically falls.
In this way, in the wafer flattening process of the related art, due to the difference in the conditions between the inside portion and the outer peripheral portion of the wafer W, the outer peripheral portion of the wafer W remains as shown in FIG. 19 and the fall in the total thickness value (TTV) of the wafer W becomes a problem.